This invention relates to speed controllable three phase drives in general and more particular to an improved speed controllable three phase drive of the type having an asynchronous motor with a slip ring rotor coupled to a three phase network and including a d.c. motor and fed through an uncontrolled rectifier connected to the slip rings of the asynchronous motor.
Arrangements of this general type referred to as three phase to d.c. mechine cascases are known, for example, from Siemens-Zeitschrift 1962, pages 710 to 714. In such cascades the speed of the drive is controlled through control of the excitation of the d.c. machine. As a result, at the maximum possible speed of the drive, the d.c. machine no longer delivers power. Thus, the asynchronous machine must always be designed for the full power. Since the asynchronous machine generates a high rotor at low speeds, while the d.c. motor generates a low counter emf, this known cascade arrangement is suitable only for relatively small speed ranges.
Another cascade is disclosed in ETZ, edition A, vol. 82, 1961, pages 589-596, and is referred to as a subsynchronous static converter. In this device, the slip power of the asynchronous motor is fed back into the three-phase network through a controlled static converter. With such a subsynchronous static converter cascade it is possible to obtain a relatively large speed range but at a corresponding large cost. The reactive power of the subsynchronous static converter cascade, which is made up of the reactive power of the machine, the comutation reactive power of the rectifier and the control reactive power of the static converter is, however, the greatest when compared with all other speed controllable drives.
In view of these deficiencies in prior art drives the need for an improved speed controllable three phase drive of the general type mentioned above which can be implemented at low cost and will result in the lowest possible reactive power requirement and which is capable of operating over a large speed range becomes evident.